The present invention relates to a radio receiver, and more particularly to a portable receiver which may be called a pager receiver and in which a plurality of frequencies can be switched.
The pager receiver can be easily carried and thus it is desired to develop a system capable of receiving a radio signal continuously by switching a frequency in any location even if the pager receiver moves to various remote locations in a manner similar to an automobile telephone.
FIG. 1 shows a structure of a portable receiver which has been used and comprises an antenna 1, a mixing unit 2, a local oscillator 3, an intermediate frequency/low frequency processing unit 4, controller 5, and a display portion 6. The output of the local oscillator 3 is provided to a mixing unit 2 to convert a signal received from an antenna 1 to an intermediate frequency signal. The intermediate frequency signal is demodulated by the intermediate frequency/low frequency processing unit 4 and the demodulated signal is displayed on the display portion 6 through the control unit 5.
In order to provide a single oscillation frequency of the local oscillator, this example adapts a zero .intermediate frequency system in which the oscillation frequency of the local oscillator 3 is equal to the frequency of the received carrier. In this case, as it is not known whether the frequency modulation component of the received signal appears on the plus side or on the minus side with regard to a carrier frequency, in the mixing unit 2, two mixers receive the signal from a common antenna. A mixer 22 mixes the output of the local oscillator 3 with the received signal and a mixer 21 mixes the output of the local oscillator 3 with the signal whose phase is shifted by .pi./2 from the output of the local oscillator 3.
Mixers 21 and 22 output the frequency deviation signal is the input difference component (and the input sum component) and the intermediate frequency/low frequency processing unit 4 demodulates the frequency deviation signal. Control unit 5 receives the demodulated data and determines the data to be displayed, thereby displaying a message from the transmitting side on the display unit 6.
Generally speaking, a crystal oscillator is used for the local oscillator 3 used in a conventional portable receiver. A single crystal oscillator is provided corresponding to a single frequency. When the local oscillator 3 adopts a multi-frequency switching system, that digital synthesizer system of a phase lock loop (PLL) method which is mainly used in a radio receiver provided in a car or in a portable radio receiver is firstly considered.
As shown in FIG. 2, the synthesizer system compares a reference signal obtained from the reference signal generator 11 with a frequency signal channel-designated by a programmable divider 12 in a phase comparator (PD) 13, and the phase difference output from the phase comparator (PD) 13 is applied to the voltage control oscillator (VCO) 15 through a low pass filter 14 so that the control voltage of the voltage control oscillator 15 is controlled , thereby varying. The oscillation frequency of the voltage control oscillator 15. The output of the voltage control oscillator 15 is applied to the programmable divider 12, thereby forming a PLL circuit. Namely the PLL circuit operates so that 1/N (N is equal to a dividing ratio of the programmable divider 12) of the oscillation frequency of the voltage control oscillator 15 becomes equal to the generation frequency generated from the reference signal generator 11. Therefore, the PLL circuit outputs N times the generation frequency of the reference signal generator 11 as the local oscillation output.
Based on this synthesizer system, a pager receiver in which the frequency is switchable can be realized, thereby extremely improving the flexibility of the received frequencies. This synthesizer system is not suitable for a portable receiver that receives signals intermittently because power consumption increases with the speed of the operation and it takes time to start-up the receiver. This is because a certain time is required for the PLL circuit to output the same frequency signal as the frequency of the reference signal generator 11. Further, the digital synthesizer has difficulty in operating at low voltages because it uses a digital divider.
On the other hand, in the conventional analog oscillator, shown in FIG. 3, i (i=1 to n) channels contain a crystal oscillator X.sub.i, diode D.sub.i and an input resistor R.sub.i, respectively. A voltage is applied to an input resistor of the designated channel, the other terminal of the corresponding crystal oscillator is grounded and thus the oscillator 3 oscillates at the frequency of the crystal oscillator X.sub.i. The oscillator 3 can sufficiently operate even at a low voltage but it requires many crystal oscillators, thereby increasing the cost of the analog oscillator and occupying a relatively larger space.